Upon taking an image of a chest region or an abdominal region by the use of the MRI device, it is often found to be a problem that an artifact occurs due to respiratory movement. There are methods for preventing the artifact caused by the respiratory movement, such as a breath-hold imaging method and a method using a navigator sequence, as described in the patent document 1, for instance.
The breath-hold imaging is the simplest method, and it is in widespread clinical use. In the breath-hold imaging, the imaging time for one time is limited to a duration that allows suspension of respirations (around 15 seconds at longest). Therefore, in the case of multi-slice imaging, or the like, which requires long imaging time, the breath-hold imaging is separated into multiple imaging operations.
There are known following methods which use the navigator sequence; a method for monitoring a position of the diaphragm without breath holding in the navigator sequence and an imaging sequence is executed during a period when the position of the diaphragm is included in a predetermined gate window; and a method for monitoring the position of diaphragm in the navigator sequence, and when the diaphragm reaches a predetermined position, the subject is instructed to hold breathing, and the imaging sequence is executed during the breath holding period. Also in the cases above, the imaging sequence is executed, being separated into multiple operations, until acquiring a predetermined number of echo signals that are necessary for reconstructing an image.
The patent document 1 further discloses a method for executing the imaging sequence after it is confirmed that the position of the diaphragm being detected in the navigator sequence has entered the predetermined gate window, a method for executing a simplified body motion monitoring even during the imaging sequence to check whether or not the position of the diaphragm is within the gate window, and a method for detecting the position of the diaphragm not only before the imaging sequence but also immediately thereafter so as to check whether or not the position of the diaphragm is within the gate window. This configuration prevents execution of the imaging sequence outside the gate window, thereby enhancing efficiency in acquiring echo signals.
In addition, echo signals of the imaging sequence that is executed outside the gate window are discarded and they are not used for reconstructing the image. The patent document 1 further discloses that, in combination with the methods described above, if there is any displacement of the position of diaphragm immediately before the imaging sequence, between the first imaging sequence and the second imaging sequence, the slice position is modified according to the displacement, so as to take an image of the same position.
The patent document 1 further discloses a method for monitoring respiratory movement by an exterior sensor instead of the navigator sequence, and executing the imaging sequence when breath holding (suspension of respiratory movement) is confirmed. Since this method uses the exterior sensor, it is possible to monitor the respiratory movement even during the imaging sequence, and an alarm display may be provided upon detecting unsteadiness in the respiratory movement.
There are examples such as the patent document 2 and the patent document 3 which disclose a technique for detecting an amount of positional displacement upon breath holding, and according to this displacement amount, a slice position, and the like, are modified upon imaging.